The present invention relates to a self-oscillation type resonant flyback converter utilizing charge stored effect of the P-N diode. The switching action of the switching transistor is done at zero voltage through the selections of a reverse recovery time of said delay diode and power diode, respectively. Due to the simplicity of the circuit, i.e., very few components, and its high reliability, it can be implemented using high-density packaging techniques. Such a resonant converter is useful in high-voltage, high-power dc-to-dc converters for distributed power supply systems.
A ringing choke converter has been often utilized as a self-oscillation type power supply. Referring to FIG. 3, a conventional ringing choke converter comprises a transformer with a primary winding inductance Lpri, a secondary winding inductance Lsec and a feedback winding inductance Lfb. A primary-side switching transistor Q1 is connected in series with the primary winding Lpri across an input dc voltage Ein and an input filter capacitor Cin, a snubber capacitor Cs is connected in parallel with switching transistor Q1. A secondary-side power diode D1 is connected in series with the secondary winding Lsec across an output dc voltage Eout and an output filter capacitor Cout. Between the feedback winding Lfb and the base of the switching transistor Q1 a current-limiting resistor R1, a speed-up capacitor C1, and a diode D2 are connected in FIG. 3.
When switching transistor Q1 is ON, energy is stored in primary winding Lpri. When switching transistor Q1 is OFF, stored energy is released through a secondary winding Lsec. The output voltage is regulated by controlling the ON and OFF times of switching transistor Q1. When the switching transistor Q1 is turned off, because of the snubber action of said snubber capacitor Cs so that the switching transistor Q1 can be controlled to achieve zero-voltage OFF switching; when the switching transistor Q1 is turned on, because of the speed-up action of the speed-up capacitor C1 so as to forward bias the base of the switching transistor Q1 that is controlled to form high-voltage ON switching. Because of discharge of the snubber capacitor Cs, causing a current spike, the product of the collector current and the voltage between the collector and emitter causes a large switching loss at the time when the switching transistor Q1 is turned on, especially in case of high output voltage and under high power conditions.
The design of a conventional flyback converter (square wave systems) is valid, but not in its integrity. A resonant flyback converter is implemented by an accurate design of its driving circuit.
In view of the foregoing, it is an object of the present invention to provide a self-oscillation type resonant flyback converter that overcomes the above-described drawbacks of prior art without modifying topology. It is a further object of the present invention to provide a self-oscillation type resonant flyback converter that facilitates soft switching, at arbitrary frequencies, has an improved efficiency and could offer a larger power than conventional flybak converter.
A self-oscillation type resonant flyback converter, comprising a transformer, a single primary-side switching transistor, a power diode and a delay driving circuit, is controlled to operate in a xe2x80x9cnaturalxe2x80x9d zero-voltage switching mode. By selecting the component values appropriately, said switching transistor could be controlled to achieve zero-voltage OFF switching and zero-voltage ON switching.
The operation of the resonant converter is such that during any non-conducting period of the switching transistor and once the power diode is in the conduction state, the delay diode also starts a conducting period, the current of the delay diode flows through a constant-current diode (reverse conducting), a capacitor and a feedback winding. When the power diode stops conduction, the delay diode becomes in reverse conduction state due to its charge stored effect more than that of the power diode, the switching transistor is still in a non-conducting state (reverse biased the base), the resonant circuit formed by the snubber capacitor and a inductance of the transformer reduces to zero/valley the voltage across between terminals of the snubber capacitor, at the moment the delay diode stops reverse conduction, the switching transistor starts conduction.
In order to boost the performance of resonant converter which accepts a broad range of input voltage and guarantees switching at zero voltage of the switching transistor. By proper selection of the reverse recovery time of the power diode, producing reverse recovery current of the power diode through the secondary winding and the output capacitor before a free resonance period starts, adding to energy of resonant circuit, forming effective to discharge an electric charge of said snubber capacitor.
In brief, the resonant converter guarantees switching at zero/valley voltage depending on the delay diode""s characteristic, and guarantees switching at zero voltage with a broad range of input voltage depending on the power diode""s characteristic.